I know this thread is mega-old, but it worked like a charm for me on a new install of Mint 15 MATE 64-bit

My system would hang on shutdown. It would go through the "system is going down for a halt NOW! going to run level..." etc. and the screen would blank, but the computer would stay powered on (fans running, lights on) until manually holding the power button. After adding the above acpi_osi=Linux, halt works as expected!

A couple things though:

I recommend creating a backup of important files (any file you need to sudo edit) before editing